Developmental changes in cardiac sarcolemmal ion regulation

Ion regulation at the sarcolemmal membrane (SL) may account for some of the difference in cardiac contractility between adult and neonate. Vesicles enriched in SL were prepared from adult and 3-5 day old canine left ventricle by homogenization and differential centrifugation. Ouabain-sensitive Na+-K+ ATPase activity was assayed using a linked-enzyme system and 3H-ouabain binding was measured in 2.5mM ATP, 2.5mM MgCl2, and 100mM NaCl. Na-Ca exchange was measured with Na-loaded vesicles in 160mM KCl and 40μM CaCl2; NaCl replaced KCl for a passive uptake measurement. Maximum ouabain-sensitive Na+-K+ ATPase activity was significantly higher in the adult (79.2 μmol/mg/hr) than in the pup (40.1). ED50's for ouabain as determined by profit analysis were not different in the two age groups (log dose: adult = -5.7 +/- 0.3; pup = -5.5 +/- 1.9). Specific binding of 3H-ouabain in adult SL was higher than in pup SL at 20 minutes incubation (adult = 152.1 pmol/mg; pup = 95.9). Ca2+ uptake in KCl was greater in adult SL (112.2 nmol/mg) than in pup (60.5) while passive uptake was not significantly different in the two groups. The authors conclude that SL ion regulatory mechanisms vary with age and hence are likely to affect the intracellular Ca2+ levels involved in cardiac contractility